In recent years, rechargeable and dischargeable electric storage devices such as battery cells (e.g., a lithium ion battery cell and a nickel-metal hydride battery cell) and capacitors (e.g., an electric double layer capacitor) have been adopted as the power sources of vehicles (e.g., an automobile and a motorcycle) and various devices (e.g., a portable terminal and a laptop computer). For example, a battery cell includes an electrode assembly and a case composed of a case body which houses the electrode assembly and a cover plate which covers an opening of the case body. An external terminal is arranged at an outer surface of the cover plate, a current collector is connected to the electrode assembly, the current collector is arranged inside the case, and the external terminal and the current collector are directly or indirectly connected to each other. With this configuration, the external terminal and the electrode assembly are electrically connected.
There are two main methods for connecting an external terminal and a current collector. One is to directly connect an external terminal and a current collector, and the other is to indirectly connect an external terminal and a current collector via an auxiliary terminal and a connecting conductor.
The former method uses an external terminal which is composed of a body, a caulking part projecting from a lower surface of the body, and a male thread part projecting from an upper surface of the body. The external terminal is arranged at an outer surface of a cover plate via a sealing member, the caulking part of the external terminal is inserted into a through-hole of a current collector inside a case, and an end portion of the caulking part projecting downward from the through-hole is caulked from below. With this configuration, the external terminal and the current collector are directly connected to each other (Patent Document 1: JP-A-2001-357833).
The latter method uses an auxiliary terminal composed of a body, a first caulking part projecting from a lower surface of the body, and a second caulking part projecting from an upper surface of the body, an external terminal composed of a head and a male thread part projecting from an upper surface of the head, and a connecting conductor including through-holes into which the second caulking part of the auxiliary terminal and the male thread part of the external terminal are to be inserted. The auxiliary terminal is arranged at an outer surface of a cover plate via a sealing member, the first caulking part of the auxiliary terminal is inserted into a through-hole of a current collector inside a case, and an end portion of the first caulking part projecting downward from the through-hole is caulked from below. The external terminal is arranged at the outer surface of the cover plate via the sealing member, the second caulking part of the auxiliary terminal and the male thread part of the external terminal are inserted into the through-holes of the connecting conductor, and an end portion of the second caulking part projecting upward from the through-hole is caulked from above. With this configuration, the external terminal and the current collector are indirectly connected via the auxiliary terminal and the connecting conductor (Patent Document 2: Japanese Patent Laid-Open No. 2003-346774).
In either of the methods, a crimp contact of a lead wire of an external device fits on the male thread part of the external terminal, and the male thread part is fixed with a nut. With this configuration, the crimp contact of the lead wire is electrically connected to an electrode assembly, which causes the external device to be electrically connected to a battery cell.
According to the latter method, since the external terminal and the auxiliary terminal are separated from each other, rotational torque applied to the external terminal when the nut is tightened is not transmitted to the auxiliary terminal. For this reason, a situation does not occur in which rotation of the auxiliary terminal causes the auxiliary terminal and a connection part of the current collector fixed by caulking to come loose to impair the connection therebetween. The latter method is superior in this respect to the former method.
However, even in the latter method, rotational torque applied to the external terminal when the nut is tightened is directly transmitted to the sealing member. Accordingly, sealing with the sealing member may be impaired. In order to solve the problem, a structure including a terminal retainer provided separately from a sealing member and an external terminal arranged at an outer surface of a cover plate via the terminal retainer has been proposed (Patent Document 3: Japanese Patent Laid-Open No. 2010-97764).
Not only the sealing member disclosed in Patent Document 1 (“the upper gasket plate 6” in Patent Document 1) and the sealing member disclosed in Patent Document 2 (“the insulating sealer 7” in Patent Document 2) but also the terminal retainer disclosed in Patent Document 3 (“the retainer 10” in Patent Document 3) receives torque from the external terminal as it stops rotation of the external terminal when a nut is tightened. Accordingly, the sealing members and the terminal retainer (which are each made of plastic and are collectively called “plastic members”) each need to be securely fixed to the outer surface of the cover plate to overcome rotational torque from the external terminal.
In this respect, Patent Document 3 discloses the process of forming the terminal retainer by finishing a part where the terminal retainer is fastened of a metal surface at the outer surface of the cover plate so as to have microscopic asperities, arranging a mold to surround the part, pouring molten resin into the mold, and solidifying the resin and the process of directly injection molding the terminal retainer at the outer surface of the cover plate (cf., paragraph [0036]).
However, these methods involve complicated work processes and high cost. The same applies to capacitors (e.g., an electric double layer capacitor).